DNS connectivity checking Introduction Name resolution (getting an IP number associate with a name) and reverse mapping (getting a name from an IP number) are at the heart of _T_C_P_/_I_P networking. A broken or erratic _D_N_S is often the cause of most network problems and sometime bad performances. 11.. IItt iiss ssllooww bbuutt iitt wwoorrkkss!! _T_C_P_/_I_P was design to operate over wide area networks as well a smaller ones. On large networks, you may from time to time encounter long delays. This is a normal (annoying sometime) conditition and the _I_P protocols are ready to cope with this. A bad DNS or broken network connectivity making the DNS unreachable will cause all kinds of delays (several seconds). Fortunatly (or unfortunatly), quite often a backup facility exists (a second DNS entry for one) and after some timeout, the system goes on. Unfortunatly, this often hides the real problem and slowness is attributed to the wrong reason. It is attributed to the _I_n_t_e_r_n_e_t which is overloaded, or the _I_S_P which is selling too much. 22.. TThhee ssttrraatteeggyy _L_i_n_u_x_c_o_n_f makes a small check with the _D_N_S before trying to configure further the machine. It probes the _D_N_S and wait for 4 seconds. It then report an error if the _D_N_S has failed to answer within this period. Here are different idea supporting this strategy. · You should have good network connectivity with your _D_N_S. The DNS may be either located on the local ethernet network, or right on the other side of a _P_P_P link. · If no DNS is available near your (network wise), you are better to setup a caching DNS on your machine. Linuxconf does this automaticly when you create a DNS with it. · _D_N_S problems are probably the most difficult problems to track or identify for new users. Better try to detect the problem soon. · _D_N_S do not fail often. When something is wrong there, the effect is so important, that the user believes is machine his either hung or broken. 33.. TThhee qquueerryy _L_i_n_u_x_c_o_n_f send the following query to the DNS nslookup -q=soa 0.0.127.in-addr.arpa It is believed that all properly configured DNS should answer this request. It is possible that a working DNS operates without a reverse mapping for the network 127.0.0. This is a flaw and should be corrected. (Run linuxconf on this machine and configure the DNS with it and linuxconf will silently fix this) 44.. TToo qquueerryy oorr nnoott Some machines may lack DNS connectivity simply because there is no DNS in the organisation. Linuxconf won't probe the DNS if there is no nameserver specified in /etc/resolv.conf. You can also disable this behavior altogether. Just go into the networking sub-menu and pick the NNaammee sseerrvveerr ssppeecciiffiiccaattiioonn ((DDNNSS)) entry. A check box allow you to disable this. This is the only purpose of the check box in this dialog.